It is well known in the pressure-sensitive adhesives industry that two approached have been tried to achieve a good removability, namely, the controlled coating processes and/or adhesive rheology.
If the coat weight of the adhesive is controlled within a certain range, the adhesive will exhibit a reduced peel force and will have a removable behavior for a short period of time. However, due to the inherent permanent characteristic of most adhesives, the adhesion will grow with age and become a non-removable.
An alternative is to use a discontinuous coat of adhesive segments or miscrospheres providing in effect a reduced coat weight. This approach is based on adhesion microspheres described in U.S. Pat. No. 3,691,140 to Silver, U.S. Pat. No. 4,166,152 to Baker et al, U.S. Pat. Nos. 4,495,318 and 4,598,212 to Howard, U.S. Pat. 4,810,763 to Mallya et al and adhesive segment described in European Patent 0180598, each incorporated herein by reference.
The rheological approach has been to design a high gel, low Tg adhesive which has a low tangent delta at the debonding frequency. These rheological properties enable the adhesive to behave like a soft material and have a low adhesion force during the peel test. This rheological approach, however, generates problems in the guillotine process.
In other words, good removability and good guillotinabilty are often properties which are contradictory to each other. The latter is a must requirement for all adhesives used in the offset sheeting business.
There have also been other attempts to formulate an adhesive to reduce the bonding between an adhesive and knife during the guillotine process. Polyalkalene glycols such as polyethylene glycol (PEG) has been used to achieve this property with tackified water insoluble elastomers as described in U.S. Pat. No. 4,548,845 to Parsons et al incorporated herein by reference, but phase separation and incompatibility between the adhesive system and PEG have limited its amount and usage. Silicone as described for instance in U.S. Pat. No. 4,151,319 to Sackoff and U.S. Pat. No. 4,346,189 to Laurant each incorporated herein by reference have also been suggested. The products display good guillotinability, and initial repositionability. Repositionability, however, is lost with time.
It would be desirable to provide a pressure-sensitive adhesive composition which exhibits removable and repositionable properties which do not deteriorate with age, as well as good guillotinability for use in offset printing applications.